Inorganic fillers and organic matrix materials with refractive index adaptation

ABSTRACT

The invention relates to the use of momodisperse, non-porous, spherical particles based on SiO 2 , TiO 2 , ZrO 2 , Al 2  O 3 , V 2  O 5 , Nb 2  O 5  or mixed systems thereof, which are optionally modified on the surface by covalently bonded organic groups, as fillers in organic matrix materials, the refractive index of the particles being adapted to the refractive index of the organic matrix according to the use. 
     Polymeric or polymerizable systems which comprise these particles can be used, for example, as embedding compositions for optical, electro-optical and optoelectronic components. Such embedding compositions show an improved optical homogeneity. Light-emitting diodes produced with them are distinguished, inter alia, by an improved light yield.

The invention relates to inorganic fillers and organic matrix materialswith refractive index adaptation. Organic materials in which inorganicfillers are incorporated appear in many industrial uses. In these cases,the inorganic fillers often not only have a replacement and dilutionfunction, but serve to modify the organic matrix material or impart tothese initially certain properties. In many cases, especially if opticalphenomena or effects play a role during application or use, use-specificadaptation of the optical refractive indices of filler and matrixmaterial is desirable or even necessary.

For example, in the case of optical, electro-optical and optoelectroniccomponents, such as, for example, light-emitting diodes and laserdiodes, optocouplers and photodetectors, the semiconductor structuralelements are enclosed by embedding compositions based on polymeric orpolymerizable systems, the embedding compositions having to fulfilloptical functions.

Because of the optical properties of the fillers employed to date, suchas, for example, calcium fluoride, calcium carbonate, barium sulfate,amorphous silicic acid and the like, the amount of these employed is,however, currently limited. About 25% by weight is to be regarded as astill tolerable upper limit for the customary fillers. This lies in thefact that due to the different refractive indices of the inorganicfiller and polymer matrix, the embedding composition becomes opticallymore inhomogeneous as the content of filler increases, and aconsiderable loss of scattered light occurs due to light scattering onthe inorganic particles. The refractive index of the usual epoxy resinsystems which are customary as embedding compositions for optical,electro-optical and optoelectronic components is about 1.5, that ofcalcium fluoride is 1.43 and that of spheroid particles of amorphoussilicic acid is 1.42.

Other optical inhomogeneities, which sometimes cause even greateroptical losses, arise from the fact that the inorganic particles usuallyemployed tend to agglomerate, cannot be dispersed sufficiently uniformlyin the casting resin and tend to undergo sedimentation in the non-curedstate. Highly transparent embedding compositions which are free fromscattered light are essential for uses such as in laser diodes andoptocouplers.

The state of affairs is somewhat different in the case of light-emittingdiodes for display purposes. These should have an intensive scatteredlight cone with a spatially uniform distribution of intensity coupledwith a minimum intrinsic absorption of the embedding composition in thespectral range of the light-emitting diode emission. For this purpose,diffuser materials which are intended to ensure adequate lightscattering are incorporated into the casting resin systems whichfunction as embedding compositions. The diffuser materials employed areusually the inorganic fillers already mentioned, such as calciumfluoride, calcium carbonate, barium sulfate and amorphous silicic acid.The scattered light yield depends on the amount of particles added andtheir refractive index, particle shape, particle size and particle sizedistribution. The greatest possible difference between the refractiveindices of the particle material and the polymer matrix material andparticles which are as spherical as possible and have a narrow particlesize distribution are favorable. However, the inorganic fillers usuallyemployed as diffuser materials as a rule have non-uniform particleshapes and a relatively wide particle size distribution which is notconstant from batch to batch. This leads to variations in the scatteringproperties of the light-emitting diodes and losses in the light yield.Inadequate dispersion properties and sedimentation of the particles inthe resin system also cause such adverse effects.

There was therefore a need for improved inorganic fillers for use inpolymeric or polymerizable systems, in particular in embeddingcompositions for optical, electro-optical and optoelectronic components.

It has now been found that monodisperse, non-porous, spherical particlesbased on SiO₂, TiO₂, ZrO₂, Al₂ O₃, V₂ O₅, Nb₂ O₅ or mixed systemsthereof are outstandingly suitable as fillers for such uses andadvantageously solve the problems mentioned in several respects. Thus,on the one hand, the refractive index of the particles can be adjustedin a controlled manner to the refractive index of the organic matrixaccording to the use, depending on the choice of oxides, or compositionin the case of mixed oxides, on which the inorganic particles are based,so that, for example, highly transparent materials are formed if therefractive indices are identical or, for example, highlylight-scattering materials are formed if the refractive indices differwidely. On the other hand, the presence of these oxides or mixed oxidesin the form of monodisperse, non-porous, spherical particles imparts tothe organic matrix materials to which they are added outstanding opticalproperties. Surface modification of the particles with covalently bondedorganic groups improves the dispersion properties in organic media, inparticular homogeneous distribution and incorporation into polymericmaterials, and reduces the tendency to agglomerate and undergosedimentation.

The invention thus relates to the use of monodisperse, non-porous,spherical particles based on SiO₂, TiO₂, ZrO₂, Al₂ O₃, V₂ O₅, Nb₂ O₅ ormixed systems thereof, which are optionally modified on the surface bycovalently bonded organic groups, as fillers in organic matrixmaterials, the refractive index of the particles being adapted to therefractive index of the organic matrix according to the use.

The invention particularly relates to the use of such particles asrefractive index-adapted fillers in polymeric or poiymerizable systemswhich are preferably used as embedding compositions for optical,electro-optical and optoelectronic structural elements.

The invention furthermore relates to polymeric and polymerizable systemswhich comprise these particles as fillers, their use as embeddingcompositions for optical, electro-optical and optoelectronic structuralelements and also the corresponding components.

The monodisperse, non-porous, spherical particles to be used accordingto the invention are known per se from the prior art. In principle, alloxide particles which can be obtained by hydrolytic polycondensationfrom alkoxide compounds of corresponding elements and are obtained inthe form of monodisperse compact spherical particles by this process aresuitable. The basic reaction conditions for the preparation of SiO₂particles by hydrolytic polycondensation can be found, for example, inthe publications W. St ober et al. in J. Colloid and Interface Science24, 62 (1968) and 30, 568 (1969) and U.S. Pat. No. 3,634,588. Otherparticles, such as, for example, TiO₂ or ZrO₂, can also be prepared bythis method. However, the particles thus prepared often display widestandard deviations for the particle diameters and have a certainporosity.

Reference is made to EP 0 216 278, which discloses a correspondinglydirected preparation process based on hydrolytic polycondensation, forthe preparation of highly monodisperse, non-porous, spherical SiO₂particles which have a standard deviation of not more than 5%. The coreof this process, which is preferred for the preparation of the particlesaccording to the present invention, is a two-stage procedure. In thisprocess, a sol or a suspension of primary particles is first formed byhydrolytic polycondensation of tetraalkoxysilanes in anaqueous-alcoholic-ammoniacal medium and is then brought to the desiredfinal size by metered addition of further tetraalkoxysilane.

The process according to EP 0 216 278 can be applied without reservationand with the same result to other oxides and also to mixed oxidesystems.

An appropriate process for the preparation of various metal oxides inthe form of spherical particles of narrow particle size distribution canbe found in EP 0 275 688.

A corresponding two-stage process for the preparation of different metaloxides and also mixed oxides which moreover also have glycolic groupsbonded chemically to the surface is described in EP 0 391 447.

For the use according to the invention of the monodisperse, non-porous,spherical metal oxide particles prepared by the abovementioned processas fillers in organic matrix materials, it is very largely a matter ofthe refractive index of the particles being adapted to the refractiveindex of the organic matrix according to the use.

Possible organic matrix materials are in principle all organic systemsinto which inorganic fillers are usually incorporated, in particularthose in which the fillers additionally have to fulfill functionalpurposes. Polymeric and polymerizable systems which are processed topolymeric shaped articles or other corresponding products are of primeinterest.

One of the fundamental possible uses is the production of opticallyhomogeneous, highly transparent polymeric articles. Use-specificadaptation of the refractive index is to be understood in this case asmeaning that the refractive indices of the filler and organic matrix areas far as possible identical.

The refractive indices of organic compounds and organic systems whichare transparent to light, such as, in particular, resins and polymers,are as a rule in the range between 1.3 and 1.6. The refractive index ofpolymer materials is about 1.5.

With the inorganic oxides considered here, there is a certain choice ofmaterials which have particular different refractive indices but withwhich the particular refractive index cannot be met exactly for allorganic matrix materials. Thus, for example, monodisperse, non-porous,spherical SiO₂ particles have a refractive index of 1.42, correspondingAl₂ O₃ particles have a refractive index of about 1.6, correspondingZrO₂ particles have a refractive index of about 1.95 and correspondingTiO₂ particles have a refractive index of about 2.3.

Particles based on mixed oxides are the choice according to theinvention for establishing other refractive index values. By combiningpreferably two or even more different oxides, particular refractiveindex values can be realized in a controlled manner. As a roughguideline for the resulting refractive index of such a mixed oxidesystem, the refractive index of the mixed oxide particles essentiallyresults arithmetically from the refractive index of the pure oxides andtheir proportional ratio in the mixed oxide particle.

Such highly monodisperse, non-porous, spherical mixed oxide particlesare prepared by the method described above, corresponding mixtures ofalkoxides of the various elements being subjected to hydrolyticpolycondensation in the simplest case.

The particular actual refractive index, depending on the nature andratio of the amounts of the alkoxides employed and on the operatingconditions chosen, can be easily determined or predetermined by routinein-process controls and/or preliminary empirical experiments.

For example, monodisperse, non-porous, spherical mixed oxide particlesin which SiO₂ and TiO₂ are present in a ratio of approximately 80:20 andwhich have a refractive index of 1.52 can be obtained from a mixture oftetraethoxysilane and tetrabutoxytitanium in a molar ratio of 4:1. Thisrefractive index furthermore corresponds exactly to that of acommercially available epoxy resin system which is often used as anembedding composition for optical, electro-optical and optoelectronicstructural elements. The corresponding polycondensation of a mixture oftetraethoxysilane and tetrapropoxyzirconium in a molar ratio of 1:1produces particles in which SiO₂ and ZrO₂ are present in a ratio ofapproximately 50:50 and which have a refractive index, corresponding tomathematical prediction, of 1.7.

In addition to metal oxide particles having a homogeneous distributionof the various oxides, mixed-phase particles can also be prepared. Thisis possible in a simple manner and by the two-stage process describedabove, for example, by forming primary particles from one oxide and thendepositing another oxide or an oxide mixture in an epitaxial growthstep. If the amount of oxide or mixed oxide formed in the epitaxialgrowth step predominates in the particle, this content largelydetermines the resulting refractive index. Here also, the dependence ofthe refractive index can be determined empirically by simple routineexperiments.

Organic modification of the surface of the particles which may beadvantageous for the particular intended use can be carried out incomplete agreement with methods such as are known for the preparation ofsilica gels customary as chromatographic sorbents. The customarymodifying agents are organoalkoxysilanes, such as, for example,methyltriethoxysilane, ethyltriethoxysilane, octyltriethoxysilane,octadecyltriethoxysilane and mono-or polyfluoroalkyltriethoxysilane, orsilanes having functionalized organic groups which allow later furthermodification by covalent bonding in a known manner. In the latter case,those organoalkoxysilanes which contain functional groups with whichcovalent bonding into the polymer material can be achieved are preferredas fillers in polymeric or polymerizable systems in respect of the useof the particles according to the invention. Examples of theseorganoalkoxysilanes are trimethoxyvinylsilane, triethoxyvinylsilane and3-glycidoxypropyltrimethoxysilane,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and[2-(3,4-epoxy-4-methylcyclohexyl)propyl]methyldiethoxysilane, as well assilanes with inorganic radicals carrying hydroxyl, carboxyl, epoxide andcarboxylic acid ester groups.

Particles having such a modification on the surface are capable ofparticipating, by means of the functional groups, in the reaction whicheffects curing of epoxy resin systems and of thereby being incorporatedcovalently into the polymer.

This organic modification can of course be carried out completelyanalogously with oxides other than SiO₂ and/or with correspondingorganoalkoxides other [sic] elements.

From EP 0 216 278 mentioned first, it can also be seen that theparticles can be modified organically by using organoalkoxysilanes inthe matrix. Reference is made to the publication mentioned for furtherdetails in this context.

In the case of modification of the surface of the particles bycovalently bonded organic groups, the properties of the particles inrespect of spherical shape, non-porosity, monodispersity and refractiveindex are not influenced, while the advantageous properties associatedwith such modification can be perceived.

The particles can be advantageously employed according to the inventionas fillers in organic matrix materials in all instances whereuse-specific adaptation of the refractive index coupled with maximumoptical homogeneity is important. The particles to be employed accordingto the invention offer advantages through the possibility of controlledadaptation of the refractive index and extreme optical homogeneity ofthe matrix materials to which they are added, due to the presence of theparticles in a highly monodisperse, compact and spherical form. Thedispersion properties of the particles in organic media are decidedlyimproved and their tendency to undergo sedimentation is reduced byorganic modification of the surface, a high uniform distributionresulting. The latter can be increased further by modification of thesurface with groups which are capable of covalent bonding to the organicmatrix material.

Examples of such uses are optically homogeneous, highly transparentpolymeric shaped articles for optical purposes, such as, for example,optical components or polymeric embedding of semiconductor elements ofelectro-optical and optoelectronic components, in particular of laserdiodes, optocouplers and photodetectors.

Particles having diameters of 50 to 1500 nm are employed for the usesmentioned. Particles in the size range from 100 to 1000 nm arepreferred.

The polymer systems serving as matrix materials for these uses, theirpreparation and their processing are known per se and can easily befound from the relevant prior art.

Examples which may be mentioned of typical epoxy resin systems which aresuitable as grit [sic] resins for embedding optoelectronic structuralelements are those which are based, for example, on3,4-epoxycyclohexanemethyl 3,4 -epoxycyclohexanecarboxylate ##STR1## oron "bisphenol A epoxide" ##STR2## Corresponding commercially availablegrit [sic] resin systems usually also comprise curing agents and curingaccelerators based on organic acid anhydrides and organic amines and, ifappropriate, other additives in the mixture or as components to be addedseparately.

The particles according to the invention, preferably having an organicmodification of the surface by means of2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, are mixed with thecorresponding grit [sic] resin in the desired amount and dispersedhomogeneously therein. The particles can be present in the polymermaterial in an amount of 0.1 to about 40% by weight, depending on theintended use and function. The customary range for embeddingcompositions for optoelectronic components is 0.5-25% by weight, basedon the total grit [sic] resin system. For embedding compositions forlight-emitting diodes in which the particles have a diffuser function,their content can vary between 1 and 15% by weight.

Because of the particular advantages of the particles to be employedaccording to the invention, their content in the corresponding materialscan also be increased significantly as required. The risk of stressesand cracking, which may threaten in the case of exposure to heat duringoperation of such components, can be reduced considerably in this way.

The particles can already be added as such to the polymer compositionsand. dispersed therein. Preferably, however, they are used in the formof a predispersion of the particles in a solvent compatible with thepolymer system. Examples of dispersion media which can be employed,depending on the nature of the polymer system, are aliphatic alcohols,such as ethanol and butanol, acetone, glycol, diethylene glycol andpolyglycols. Dispersion in polyglycols which are solid at roomtemperature is effected by heating to above the melting point.Predispersion of the particles as a "masterbatch" in the correspondingpolymer system or a resin or polymer material compatible with the systemis likewise preferred. The content of particles therein can be up toabout 50% by weight.

When the particles according to the invention are used as a fillingmaterial in embedding compositions for light-emitting diodes where theparticles have an optical diffuser function, a higher scatteringintensity, an increase in the light yield and an improvement in thehomogeneity of the scattered light cone compared with customary diffusermaterials are to be observed. The efficiency of the light-emittingdiodes is evaluated from the brightness or light yield, defined emissioncharacteristics and a homogeneous luminous pattern.

Surprisingly, it has been found that light-emitting diodes withembedding compositions which comprise the particles according to theinvention as diffusers have a scattering intensity which is about 25%higher and a light yield which is higher by a factor of about 2 comparedwith corresponding components comprising calcium fluoride as thediffuser material.

Use as fillers in highly transparent or also opaque films of plastic andin photographic emulsions is also a further possible use according tothe invention of monodisperse, non-porous, spherical oxide particles ofadapted refractive index.

In carrier films for magnetic recording tapes, these particles serve toimprove the slip properties of the tapes, preferably if they have anorganic modification on the surface.

EXAMPLE 1

A solution, adjusted to a temperature of 30° C., of 66.7 g (0.32 mol) oftetraethoxysilane and 27.2 g (0.08 mol) of tetrabutoxytitanium in 120 mlof ethanol is added all at once to a mixture, thermostaticallycontrolled at the same temperature, of 520 ml of ethanol, 230 ml ofwater and 140 ml of 25% ammonia, while mixing intensively. The reactionmixture is stirred intensively for a further 15 seconds and then left tostand for one hour.

The resulting particles are present in dispersed form in the reactionmixture and can be isolated as a dry powder by centrifugation and, ifappropriate, repeated taking up in water and renewed centrifugation andsubsequent drying.

Dense spherical particles having a particle diameter of 500±15 nm areobtained. SiO₂ and TiO₂ are present in the particles in a ratio ofapproximately 80:20. The refractive index of the powder is determined as1.52 microscopically by the known method of the shift of the Becke line.

EXAMPLE 2

A solution, adjusted to a temperature of 30° C., of 42 g (0.2 mol) oftetraethoxysilane and 65.5 g (0.2 mol) of tetrapropoxyzirconium in 150ml of ethanol is added all at once to a mixture, thermostaticallycontrolled at the same temperature, of 520 ml of ethanol, 230 ml ofwater and 140 ml of 25% ammonia, while mixing intensively. The reactionmixture is stirred intensively for a further 20 seconds and then left tostand for two hours, and is subsequently stirred again for two hours.The particles are isolated as in Example 1.

Dense spherical particles having a particle diameter of 500±20 nm areobtained. SiO₂ and ZrO₂ are present in the particles in a ratio ofapproximately 1:1. The refractive index is determined as 1.65.

EXAMPLE 3

90 ml (0.4 mol) of tetraethoxysilane are added to a mixture,thermostatically controlled at 30° C., of 600 ml of ethanol, 225 ml ofwalter and 140 ml of 25% ammonia, while mixing intensively. A dispersionof particles of average diameter 500 nm is obtained by this operation.The resulting particles are redispersed in 500 ml of ethanol. A mixtureof 9 ml (0.04 mol ) of tetraethoxytitanium and 400 ml of ethanol isadded dropwise to this dispersion at 40° C. in the course of 2 hours andthe mixture is then allowed to react for a further 2 hours. Epitaxialgrowth of TiO₂ on SiO₂ primary particles is achieved by this process.Working up is carried out in accordance with Example 1.

Dense spherical particles having a particle diameter of 500±20 nm areobtained. The refractive index is determined as 1.48.

EXAMPLE 4

The procedure is as in Example 3, except that the epitaxial growth stepis carried out with 18.25 g (0.08 mol) of tetraethoxytitanium in 400 mlof ethanol. Dense spherical particles having a particle diameter of490±15 nm and a refractive index of 1.51 are obtained.

EXAMPLE 5

The procedure is as in Example 3, except that the epitaxial growth stepis carried out with 34.2 g (0.15 mol ) of tetraethoxytitanium. Densespherical particles having a particle diameter of 555±15 nm and arefractive index of 1.58-1.60 are obtained.

EXAMPLE 6

123 ml of tetraethoxysilane are added all at once to a mixture of 825 mlof ethanol, 308 ml of water and 93 ml of 25% ammonia which has beenthermostatically controlled at exactly 35° C. The mixture is stirredvigorously, and the stirrer is switched off after 15 seconds. Thereaction mixture is allowed to stand for one hour. Thereafter, adispersion of seed particles 375 nm in size (SEM control) is present.

The seed dispersion is prepared for the epitaxial growth process byadding a further 308 ml of water and 93 ml of 25% ammonia solution.Thereafter, the dispersion is brought to a reaction temperature of 40°C., which is maintained exactly throughout the entire process.

An epitaxial growth mixture comprising 2213 ml of ethanol and 2213 ml oftetraethoxysilane is pumped continuously into the reaction mixture at ametering rate of about 7 ml/minute by means of a metering pump. Whenaddition of the epitaxial growth mixture is complete, stirring of theentire dispersion at 40° C. is continued overnight. Subsequent SEMcontrol shows particles of 1000 mm [sic] diameter.

For modification of the surface of the particles, the reaction mixtureis concentrated, all the ethanol and the ammonia being distilled off.The now aqueous suspension is redispersed in about 1800 ml of 1-butanol,the water being distilled off azeotropically.

The monospheres are silanized in the butanolic dispersion. For this, thedispersion (˜10.5 mol, based on solid SiO₂) is heated to 70° C., 10.4 g(4 mmol/mol of SiO₂) of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane areadded and the entire mixture is allowed to react at the same temperatureovernight.

2900 ml of acetone are then added to the mixture as a solubilizingagent, and 5670 g of an epoxy resin based on 3,4-epoxycyclohexanemethyl3,4-epoxycyclohexanecarboxylate (Araldit® CY 179; Ciba Geigy AG) areweighed in. Thereafter, the solvents are stripped off quantitatively; anapproximately 10% dispersion of the spherical SiO₂ particles in theepoxy resin forms. This dispersion is the starting point for processinginto, for example, epoxy casting resins for light-emitting diodes.

EXAMPLE 7

Light-emitting diode semiconductor elements are cast by the customarytechnique using the casting resin system from Example 6 and the resin iscured by means of heat.

Components which comprise the corresponding amount of CaF₂ as a diffuserin the embedding material serve as a comparison.

    ______________________________________                                                         Emission angle                                                                            Brightness                                       ______________________________________                                        Component according                                                                            55°  8.5                                              to the invention                                                              Comparison component                                                                           77°  3.5                                              ______________________________________                                    

The components according to the invention show an approximately 25%higher scattering intensity and a light yield which is higher by afactor of about 2.

We claim:
 1. A method of matching the refractive index of a filler tothat of an organic matrix in which it is contained comprising a)preparing monodisperse, non-porous, spherical particles based on mixedoxides by combining two or more oxides selected from the groupconsisting of SiO₂, TiO₂, ZrO₂, Al₂ O₃, V₂ O₅ and Nb₂ O₅ having arefractive index largely determined by the refractive indices of theoxides and their proportional ratio within said particles, b) optionallymodifying the surfaces of said particles with covalently bonded organicgroups, and c) incorporating said particles into organic polymeric ororganic polymerizable systems, the refractive index of said particlesbased on mixed oxides being adapted to the refractive index of theorganic matrix according to the desired characteristics of the intendeduse thereof.
 2. A method according to claim 1, wherein the refractiveindex of said particles corresponds arithmetically to the refractiveindices of the oxides in the proportion within said particles.
 3. Amethod according to claim 1, wherein particles are organic matrix withsubstantially identical refractive indices and used to provide opticallyhomogeneous, highly transparent polymeric shaped articles showingrefractive indices in the range between 1.3 and 1.6.
 4. A methodaccording to claim 1, wherein particles and organic matrices with widelydifferent refractive indices are used to provide highly light-scatteringpolymeric shaped articles.
 5. A method according to claim 3 whichprovides optical, electro-optical or optoelectronic components.
 6. Amethod of claim 1 wherein the index of the particles is matched to thatof said matrix.
 7. A method of claim 1 wherein the index of theparticles is selected to be significantly different from that of saidmatrix.
 8. A polymeric or polymerizable system comprising an organicmatrix having a given refractive index and an inorganic filler, whereinthe filler comprises preformed monodisperse, non-porous, sphericalparticles based on two or more oxides selected from group consisting ofSiO₂, TiO₂, ZrO₂, Al₂ O₃, V₂ O₅ and Nb₂ O₅ having a refractive indexlargely determined by the refractive indices of the oxides and theirproportional ratio within said particles, said filler having arefractive index which is adapted to the refractive index of the organicmatrix according to the desired characteristics of the intended usethereof.
 9. A system according to claim 8, wherein said particles aremodified on their surface by covalently bonded organic groups.
 10. Asystem according to claim 9, wherein said particles are in covalentlybonded form.
 11. A system according to claim 8 which is an embeddingcomposition for an optical, electro-optical or optoelectronic structuralelement.
 12. An optical, electro-optical or optoelectronic component,comprising a system according to claim 8 as an embedding composition.13. A system of claim 8 wherein the index of the particles is matched tothat of said matrix.
 14. A system of claim 8 wherein the index of theparticles is selected to be significantly different from that of saidmatrix.
 15. A component of claim 12 which is a light emitting diode andwherein the index of the particles is selected to be significantlydifferent from that of said matrix.